This invention relates to the post-weld heat treatment of thin wall metal structures. In particular the invention relates to a method of locally heat treating a weld seam without thermally affecting material adjacent to the weld.
The shells of typical satellite fuel and oxidizer tanks and liners of composite over-wrapped pressure vessels (COPV) for pressurant tanks are fabricated by welding metal domes or domes and cylinders together. Of many important design criteria, an overriding design driver is the need for low mass. The need for reduced mass drives the material choices and wall thickness of the tank design. In the areas of the welds that hold the domes and/or cylinders together, the thickness is often thicker than the rest of the tank because of the reduced available material strength caused by the effects of the welding process on the structure. Some of the local strength reductions are caused by increased grain size and other microstructural changes, loss of temper, and residual strain in the recast material. Some of these strength reductions can be alleviated through the use of post-weld heat treatments.
In a typical all metal satellite tank made of titanium, the tank may be subjected to a vacuum stress relief operation for approximately 2 hours at approximately 1000° F. (538° C.). The process requires significant expense in equipment, such as a vacuum furnace, special tooling, and handling and protection, wherein the tanks are wrapped with a number of layers of metal foil. In total, the process requires approximately 24 hours of time to accomplish. There are significant risks in returning the furnace chamber to atmospheric pressure because the tanks have small ports, are not rated for a negative pressure, and may collapse.
Alternatively the tanks may be stress relieved in an inert atmosphere or by using a retort. The disadvantages of a retort are largely in cost and time. Typical process times are greater than 40 hours and have proven to be significantly more costly than vacuum stress relief operations. Inert atmosphere heat treatments are generally used on larger tanks that do not fit in available vacuum furnaces.
In certain tanks, elastomeric bladders may be installed inside the tank shells. The bladders cannot survive the temperature of a post-weld heat treatment. As a result, the tank shell thickness and resulting mass must be increased to maintain strength margins caused by the reduced material properties at the weld locations.